1. Field of the Invention
This invention relates to a magnetooptical recording medium with which writing, reading, and erasing of information are performed through application of a laser beam.
2. Description of the Prior Art
In recent years, a substantial amount of effort has been directed toward the development of an optical recording medium which satisfies various requirements including high density, large capacity and high speed access.
Of a wide range of optical recording media, magnetooptical recording medium is most attractive due to its unique advantage that information can be erased after use and new information can be written thereon (see U.S. Pat. No. 3,965,463).
The magnetooptical recording medium comprises a glass or plastic disk substrate and a perpendicular magnetic tion layer as a recording material which is formed on the substrate.
In general the recording medium has a concentric circular or spiral recording track, the direction of magnetization of which track is entirely forced to be for example downward (or upward) by a strong external magnetic field before writing an information. An information can be written with a presence and/or length of pit having a reversed, for example upward (or downward), magnetization against the primary direction of magnetization.
Principle of pits formation
In the pits formation, a feature of laser, superb coherence in space and time, are advantageously used to focus a beam into a spot as small as the diffraction limit determined by the wavelength of the laser light. The focused light is applied to the disk surface to write data by producing pits less than 1 .mu.m in diameter on the surface, or to retrieve the stored information from a video or audio disk. In the optical recording, a recording density up to around 10.sup.8 bit/cm.sup.2 can be theoretically attained, since a light beam can be concentrated into a spot with a diameter as small as its wavelength.
As shown in FIG. 1, in a magnetooptical recording, a laser beam (L) is focused onto a recording layer 2 to heat its surface while a bias magnetic field (Hb) is externally applied, so that magnetization (M) in the locally heated surface area can be aligned in the direction of the bias magnetic field. As a result, reversely magnetized pits (P) are formed. The magnetic field strength required to reverse the magnetization (M) in a recording layer 2, the minimum magnetic field strength that is coersive force (Hc) capable of forming pits varies with temperature; generally this field strength (Hc), decreases as the temperature increases.
Even a weak magnetic field otherwise unsuitable for pits formation at room temperature can thus be used for recording if the recording layer 2 is heated to lower Hc.
Ferromagnetic and ferrimagnetic materials differ in the temperature dependencies of M and Hc. Ferromagnetic materials have Hc which decreases around the Curie temperature, and recording is performed based on this phenomenon; thus it is referred to as Tc recording (Curie temperature recording).
By contrast, ferrimagnetic materials have a compensation temperature, below the Curie point, at which magnetization (M) becomes zero. Since Hc drastically changes around this temperature, data recording by means of light becomes possible. This process is called the Tcomp. recording (compensation temperature recording). However, it is possible to perform recording on ferrimagnetic materials by Tc recording.
Principle of reading
FIG. 2 illustrates the principle of data reading based on the magnetooptical effect. Light is an electromagnetic wave with an electromagnetic-field vector normally emanating in all directions on the plane perpendicular to the light path. When light is converted to linear polarized beams and applied onto a recording medium, it is reflected by the surface or passes through the recording layer 2; at this time the plane of polarization rotates according to the direction of magnetization (M).
For example, if the polarization plane rotates .theta..sub.k degrees for upward magnetization, it rotates -.theta..sub.k degrees for downward magnetization. Therefore, if the axis of a light analyzer is set perpendicular to the plane inclined at -.theta..sub.k degrees to the recording surface, the light reflected by a downward-magnetized surface area cannot pass through the analyzer, whereas the light reflected by an upward-magnetized pit (P) can be captured by the detector for an amount of sin (2.theta..sub.k). As a result, upward-magnetized pits show up brighter than downward-magnetized areas.
S. Tsunashima proposed a magnetooptical recording medium using a double-layered perpendicular magnetic layer (see Japanese Unexamined Published Patent Application No. 78652/1982). The magnetic layer consists of a first layer having a low Curie point (TL) and a high coercive force (HH) and a second layer having a relatively high Curie point (TH) and low coercive force (HL). The operating principles of Tsunashima's medium are as follows. Upon irradiation with a laser beam, the medium is heated to a temperature higher than the low Curie point (TL) and lower than the high Curie point (TH). This causes the coercive force of the second layer to be reduced to an appreciably low level, though it may not be zero. Then, a stronger bias magnetic field (Hb) is applied to reverse the direction of magnetization in the second layer, thereby writing data into that layer. Since the second layer initially has a low coercive force (HL), a strong bias magnetic field need not be used to write data into this layer. Furthermore, the medium need not be heated up to the high Curie point (TH), but simply to a temperature higher than the low Curie point (TL), and this permits the use of a low-power laser for data write.
When the laser beam is removed, the medium cools down rapidly. As soon as the temperature of the first layer decreases below TL, the magnetization in the first layer is reversed by the already reversed magnetization in the second layer since the coercive force of the first layer still remains zero. This completes the writing of data in the first layer, which is generally referred to as TC writing. The recorded information is preserved by the first layer having high coercive force (HH), because even if a great external magnetizing force (He) comes near the medium accidentally, the magnetization in the first layer will not be reversed again, thereby preventing the loss of the already formed pits (erasure of the recorded information). Therefore, the medium proposed by S. Tsunashima depends principally on the first layer for data recording and is classified as a TC writing system.
In reproduction mode, a stronger light should be used since the S/N ratio of the reproduced information is proportional to I or its square root, I being the intensity of the reproduction light. However, the greater the intensity of the light, the more elevated the temperature of the medium. The value of .theta..sub.k is decreased as the temperature of the medium becomes closer to the Curie point. Since the S/N ratio is generally in proportion to .theta..sub.k, the intensity (I) of the reproduction light may not be rendered too great if one wants to achieve a high S/N ratio. Particular care must be taken to reproduce data at a temperature (TR) lower than TL since the coercive force of the first layer becomes zero if its temperature is at TL. The value of .theta..sub.k of the first layer may be reduced, but the second layer whose Curie point (TH) is higher than TL retains a satisfactorily high .theta..sub.k. Therefore, information can be reproduced from Tsunashima's medium at a TR higher than allowed for a single-layered medium without suffering from a decrease in .theta..sub.k. This permits the use of a reproduction light having a correspondingly enhanced intensity, thereby providing an increased S/N ratio (.varies..sqroot.I.times..theta..sub.k or I.times..theta..sub.k).
As shown above, the recording medium proposed by S. Tsunashima has excellent features. With this medium, information is temporarily recorded in the second layer before it is finally transferred to (replicated in) the first layer, and for this purpose, a high interface wall energy density (.delta..omega.) is necessary. During the reproduction mode, an unwanted external magnetic filed He (e.g. leakage flux from the actuator or the recording permanent magnet in a record/reproduce pickup) may accidentally come close to pits. If the direction of the approaching external magnetic field is opposite the direction of magnetization in the pits in the second layer, such pits are erased since this second layer has a low Hc. If the direction of the external magnetic field is parallel to that of magnetization in the pits, the magnetization in the area around the pits which has been heated considerably by the laser beam is reversed by He, causing excessive enlargement of the pits. In either case, the S/N ratio drops and erroneous reproduction occurs. These troubles are absent from the first recording layer since, as already mentioned, it has a large Hc. If there exists a high interface wall energy density between the first and second layers, the pits in the second layer are protected by those in the first layer, and as a result, the former pits will be neither erased nor enlarged by He. Therefore, the interface wall energy density is desirably as high as possible so as to enable reliable recording with the magnetooptical recording medium of the double-layered structure.
In order to avoid the risk of erasure or excessive enlargement of the pits in the second layer by He, either one of the following relations 1 and 2 must be satisfied. ##EQU1## if the direction of the magnetization in the first layer is parallel to that in the second layer ##STR1## and ##EQU2## if the two directions of magnetization are anti-parallel ##STR2## In each formula, .delta.'.omega. is the interface wall energy density for read mode (when the medium is heated to TR), Ms'.sub.2 is the saturated magnetic moment of the second layer (read layer) for read mode, t.sub.2 is the thickness of the second layer, and Hc'.sub.2 is the coercive force of the second layer for read mode.
Both formulas (1) and (2) dictate that in order to avoid the risk of pit erasure or enlargement in the second layer, the interface wall energy density in read mode (.delta.'.omega.) should be as high as possible. Since the interface wall energy density decreases as the temperature of the medium increases, the formulas also suggest the favor for a high interface wall energy density (.delta..omega.) at room temperature.
The pits in the first layer should not be erased or excessively enlarged either, even if a magnet or other external magnetic field He' are accidentally brought close to the medium during storage. In order to meet this requirement, the following relation must be satisfied if the direction of magnetization in the first layer is anti-parallel to that in the second layer: ##EQU3## wherein .delta..omega. is the interface wall energy density at room temperature Ms.sub.1 is the saturated magnetic moment of the first layer (preserving layer) at room temperature, t.sub.1 is the thickness of the first layer, and Hc.sub.1 is the coercive force of the first layer at room temperature. This formula (3) also suggests that the value of .delta..omega. at room temperature is preferably as high as possible in order to avoid the risks of pit erasure or enlargement resulting from accidental external magnetic field. Suppose a medium wherein the first layer is made of TbFe with a thickness of 500 .ANG. and the second layer is made of GdFe with a thickness of 500 .ANG.. If TbFe has Hc.sub.1 of 3,000 Oe and Ms.sub.1 of 117 emu/cm.sup.3, an external magnetic field (He') of 3,342 Oe or more will be hazardous if .delta..omega. is 0.4 erg/cm.sup.2. However, if .delta..omega.=1.6 erg/cm.sup.2, no hazard will occur unless He' exceeds 4,368 Oe. This simple example will help one understand that the interface wall energy density (.delta..omega.) at room temperature is preferably as high as possible in order to avoid the risk of pits erasure or their excessive enlargement due to an accidental external magnetic field.
S. Tsunashima recommends the use of TbFe or DyFe as the material of the first layer and Gd-Fe or Gd-Co as the material of the second layer. However, the present inventors have found that none of the combinations of these materials provide an adequately high value of .delta..omega..